1. Field of the Invention
The present invention relates to a method and apparatus for facilitating the tuning of a digital television receiver with respect to a transmitted signal. More particularly, the present invention relates to a method and apparatus for permitting a vestigial sideband (VSB) demodulator to lock on to a channel for reception by controlling the starting points of the carrier tracking loop and the symbol timing recovery loop.
2. Discussion of the Related Art
Any terrestrial digital TV system must overcome a number of problems in transmitting signals to a receiver. The United States has adopted the Advanced Television System Committee (ATSC) System using eight level vestigial sideband (8-VSB) modulation as its digital television standard. The recovery of data from such modulated signals containing digital information in symbol form usually requires three functions at a receiver: timing recovery for symbol synchronization, carrier recovery (frequency demodulation to base band) and channel equalization.
Both symbol and carrier frequencies are specified in the transmission standard. However, because of hardware parameter drift in both the transmitter and receiver, the actual carrier frequency and symbol timing may vary from the specified values. The difference between the actual carrier frequency and symbol timing and the specified carrier frequency and symbol timing is termed ‘offset’. When a new channel is tuned, the presence of offsets in the carrier frequency and symbol timing must be detected and the receiver VSB demodulator circuitry adapted to receive the actual carrier and symbol frequencies.
One problem is that it may take a long time for the VSB demodulator to acquire a “lock” on a channel if the starting points of the carrier tracking loop (CTL) and symbol timing recovery (STR) loops are far away from their locked frequencies. This may occur, for example, if a previous channel had carrier and/or symbol frequency offsets at one end of the allowable range of frequencies, and the new channel has carrier and/or symbol frequency offsets at the other end of the allowable range of frequencies. This problem is aggravated when the CTL and STR loop bandwidths are set low to ensure reliable acquisition and tracking. It is even possible that lock will not be achieved if the difference is too large.
To achieve carrier recovery in accordance with adopted United States standards, a small pilot signal at the suppressed carrier frequency is added to the transmitted signal to help achieve carrier lock at a VSB receiver. This pilot tone is detected and the CTL adapted to lock to the frequency of the pilot tone.
To achieve symbol timing recovery in accordance with adopted United States standards, the symbol stream is analyzed to detect embedded synchronization signals. Each data frame comprises two fields, each field including 313 segments, each segment including 832 multilevel symbols. The first segment of each field is referred to as a field sync segment and the remaining 312 segments are referred to as data segments. Every segment includes a four-symbol segment sync sequence all having the same four-symbol value. Each field sync segment includes the segment sync sequence followed by a field sync component comprising a predetermined 511 symbol pseudorandom number (PN) sequence and three predetermined 63-symbol PN sequences. The middle PN sequence is inverted in successive fields. A VSB mode control signal (defining the VSB symbol constellation size) follows the last 63-symbol PN sequence which is followed by 96 reserved symbols and 12 symbols copied from the previous field. Each data segment also comprises the four-symbol segment sync sequence followed by 828 data symbols. The data symbols typically contain MPEG compatible data packets. The timing of the four symbol segment sync characters are detected and used to adapt the STR circuitry to lock to the symbol frequency. A full description of this system can be found in the 1994 proceedings of the National Association of Broadcasters, 48th Annual Broadcast Engineering Conference, Mar. 20–24, 1994, the contents of which are incorporated herein as a reference.
A receiver which will lock to a newly tuned channel without a long delay, and without the possibility of not locking at all is desirable. The present invention deals specifically with a method and apparatus for a more reliable and faster acquisition of symbol and carrier lock for a transmitted channel in the receiver.